You didn't say about the new content/niche that you will build for this domain. If not the same niche, you should not forget that the PR comes from relative adult pages, in order to create a non adult site those links have to be deleted and PR will decrease, IMO I wouldn't spend the money and effort because the domain has been characterized as adult, unless you want to continue in that sector.

Not sure about that. I bought an expired domain 6 months ago, put some content on it and the TBPR was PR5. The content took a day to write up. It had good backlinks and may be a good candidate for feeding a link to other sites.

It's not clear what you want to do with the domain, i.e. build adult content on it or something else, like link to your more established sites. maybe that info will help getting an answer.

Page rank does not refer to domains, refer to pages. PR stays if you respect the current site structure, or at least you need to 301 redirect to new urls. What about the link profile? is it solid? u need further research to make sure it's worth it, pr5 does not mean anything, but it may be a very good buy if you know what you're doing

I'm thinking if I rebuild the site with clean seo pages, it has to make a difference right ? --ues you are quite right that is why it is not necessary to do it as wouldn't u guys think any adult site with a PR5 alone would be worth it ? - i think iw certainly will. invest your money and be ready to feedback)